1. Field of the Invention
The present invention relates to a step-up continuous mode DC-to-DC converter with integrated fuzzy logic current control.
2. Discussion of the Related Art
The need is increasingly felt to have integrated DC-to-DC converters featuring a high performance in terms of transient load response, a wide stability range and a low cost.
This need is particularly felt in fields of application where there is a tendency to increase integration density, minimizing the number and density of the circuit components. This is the case, for example, of converters for pagers, cellular telephones, hard disks, portable equipment and the like.
The DC-to-DC converters that may be used in accordance with the present invention are known in the literature as fixed-frequency step-up converters with duty cycle control. In most cases, duty cycle control is performed by means of a simple current-mode fedback linear adjustment diagram. The discontinuous operating mode of the converter does not entail linear control stability problems, since the closed-loop transfer function of the controller is of the first-order type, owing to the drop to zero of the inductor current, during each clock cycle. At the crossing frequency (unit loop gain), the transfer function assuredly has a phase margin of more than 90.degree., ensuring its stability. Moreover, in this operating mode the transfer function does not contain terms that depend on the load.
Viceversa, in continuous-mode operation the lack of a drop to zero in the inductor current introduces a tendency of the system to respond to load transients with variations of the duty cycle which tend to destabilize the controlled system. Moreover, the use of a current-mode control diagram for continuous-mode operation introduces in the transfer function a term which depends on the output load, introducing an additional phase shift which adds to the one introduced by the output LC unit, making it extremely difficult, if not virtually impossible, to achieve optimum feedback loop compensation for a wide range of load and input conditions.
The problem is particularly evident in operating conditions close to the limits of the specifications, with a highly variable load or with low supply voltages, where a high speed of response to transients in the load and in the supply voltage is required.
The problem linked to the current-mode control of a step-up converter in continuous-mode operation is usually dealt with by resorting to the so-called "current-mode with compensation ramp" method. According to this method, as shown in the block diagram of FIG. 2, a ramp signal is generated inside the circuit and is added to a signal which is proportional to the current ramp that flows across the inductor during the first step, in order to reduce the loop gain proportionally to the steady-state value of the duty cycle. In this manner, the instability problems arising for the continuous mode when the value of the duty cycle exceeds 50% are reduced considerably.
The block diagram shown in FIG. 1 comprises, and designates with the reference numeral 1, a DC-to-DC converter which is supplied by a supply voltage 2 with an output voltage Vout. The reference numeral 3 designates an error amplifier which receives in input the output voltage Vout together with a reference voltage Vref indicated by the block 4. A block, designated by the reference numeral 5, is cascade-connected to the error amplifier 3 and indicates the gain of the error amplifier.
The output voltage of the block 5, designated by Ve, is input to an additional comparator, designated by the reference numeral 6, which receives in input the ramp signal which is proportional to the switching current during the ON step with the addition of a compensation ramp signal designated by the reference numeral 7.
The output of the amplifier 6, together with the output of an oscillator block 8, is sent to a duty cycle control logic block, designated by the reference numeral 9, the output whereof controls the DC-to-DC converter.
The transfer function of the circuit illustrated in FIG. 1 has a zero with a positive real part, the value whereof depends, among other factors, on the load conditions and on the value of the duty cycle, in addition to a pair of conjugate complex poles, which tends to generate overshooting in response to transients.
It should be noted that in the diagram of FIG. 2 the reference numeral 10 designates a load to which the voltage in output from the DC-to-DC converter is sent.
This compensation method, however, is not effective for working over a wide range of load and supply conditions and also reduces the closed-loop gain of the regulating device, consequently reducing the dynamic performance.
A second approach to the above-mentioned problems is a current-mode fuzzy logic control, which is the subject of European patent application no. 94830328.4 filed on Jul. 1, 1994 in the name of this same Applicant, which is incorporated herein by reference. This approach uses an (external) fuzzy control algorithm whose operation is based on the acquisition of the input and output voltages, of the current across the inductor and optionally of the temperature of the power transistor.
This solution, however, cannot be easily implemented on a chip. Moreover, it is difficult to acquire the instantaneous value of the inductor current.
Fuzzy control of the duty cycle and optionally of the base current of the power transistor which also utilizes the temperature measured on the power transistor can also be complex and not easily implemented.
Another drawback of the approach proposed in the above-cited patent application is that the control system, as mentioned, is inadequate as an architecture to be used in an integrated regulator, since it is based on the measurement of values which are not available during all the operating steps in low-power systems with a high degree of circuit integration, high efficiency, and a minimal number of passive components and control inputs.